Light offers many advantages when used as a medium for propagating information, the foremost of which are increased speed and bandwidth. In comparison with electrical signals, signals transmitted optically can be switched and modulated faster and can include an even greater number of separate channels multiplexed together. For these, as well as other reasons, lightwave transmission along optical fibers is widespread.
Light can be propagated through planar waveguide structures as well as optical fibers. Planar waveguide structures having a wide variety of functionalities are currently available and many new such devices and components will likely result from future research and development. These planar structures are advantageous because they can be compactly incorporated together in or on a planar platform, i.e. substrate, to form planar packages analogous to integrated circuits (ICs). These structures in general are referred to as integrated optics. Integrated optical “chips” comprise a substrate on which or in which various integrated optical components or devices are formed. Planar waveguides analogous to conductor traces in semiconductor electronic ICs that are mounted in or on the substrate are employed to guide light to various optical, electro-optical, and optoelectronic devices or components on the chip.
In many applications, it is desirable that the optical signal being transmitted through the planar waveguide structures be optically coupled into or out of the integrated optical chip. These signals may, for example, be coupled to an optical fiber that is oriented out of the plane, i.e., above or below, the planar waveguide structure via a grating coupler. The grating coupler, forming a part of the planar waveguide structure, may have a plurality of scattering elements designed to scatter light along a predetermined optical path.
In the case where the planar waveguide receives light delivered by the optical fiber, a coupling efficiency of the grating coupler may be defined as the ratio of the amount of light coupled into the planar waveguide to the quantity of the light delivered by the optical fiber. The coupling efficiency is unfavorably affected by two mechanisms: (i) reflection of light on surface of the grating coupler, and (ii) transmission of light through the grating coupler and absorption of the transmitted light by the substrate of the planar waveguide. Likewise, in the case where the planar waveguide delivers light to the optical fiber, the coupling efficiency, defined as the ratio of the amount of light coupled into the optical fiber to the quantity of the light delivered by the planar waveguide, is also affected by the two mechanisms. In both cases, the coupling efficiency affects the performance of the planar waveguide structure and, consequently, the integrated optical chips. Thus, there is a need for optical elements incorporated by the grating coupler, where the optical elements suppress the two mechanisms and enhance the coupling efficiency of the grating coupler.